Conventionally, in a power conversion apparatus for converting a DC power into a DC power, a bridge circuit is configured with semiconductor switching devices (hereinafter referred to as “switching devices”). The bridge circuit converts a DC power into an AC power or converts an AC power into a DC voltage. A power conversion apparatus with a primary side and a secondary side isolated from each other is built by using two bridge circuits and connecting the AC terminals of the two bridge circuits through a transformer. Some power conversion apparatuses for performing high-efficient DC/DC conversion employ soft switching techniques using snubber capacitors for switching devices (for example, see U.S. Pat. No. 5,027,264 (PTL 1)).
Electric power transmission systems using high-voltage direct current (which hereinafter may be referred to as HVDC) have been developed for achieving higher efficiency of electric power transmission systems. A high voltage is input to or output from a power conversion apparatus used in such HVDC electric power transmission systems. In this case, the voltage value has to be equal to or lower than the breakdown voltage of the switching devices in view of reliability. In order to handle a voltage equal to or higher than the breakdown voltage of a switching device alone, it has been proposed that a power conversion apparatus is configured by connecting a plurality of switching devices in series (for example, see Japanese Patent Laying-Open No. 2000-262068 (PTL 2) and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-528562 (PTL 3)).